The invention relates to magnetic tape read/write devices, and more particularly to methods and apparatus for dynamically positioning a magnetic tape with respect to a tape head in such a device.
Despite recent advances in techniques for storing data on high density data storage devices such as compact disks (CDs) and magnetic disks, there still exists a need to store data on magnetic tape. Such data can comprise for example a database of information, such as survey information, as well as a computer program. Magnetic tape provides a low cost alternative to other high density data storage devices. Further, data on magnetic tape can be easily erased and modified, unlike read-only CDs. An additional advantage of magnetic tape over other data storage media is that data can be recorded in analog format, as well as digital format. One example of the use of magnetic tapes is in recording data from a seismic survey.
Magnetic tapes of data are accessed by a tape processing device, which can perform one or both of storing (xe2x80x9cwritingxe2x80x9d) data onto the tape, or accessing (xe2x80x9creadingxe2x80x9d) data previously stored on the tape. A generic term for a tape processing device is a xe2x80x9ctape drivexe2x80x9d. A tape processing device comprises a tape head for one or both of reading and/or writing data from or to the magnetic tape. The tape head comprises tape head elements, which can perform one or both of these functions. Tape processing devices typically further include at least one, and typically more, guides for supporting the tape as it moves across the tape head. The guides can either be fixed or stationary guides such as spindles, or rollers which roll with the tape as the tape moves across the tape head. The guides help to align the tape with respect to the tape head. The tape support guides can also be powered rollers to rotate either in the direction of, or opposite to the direction of, the tape travel to assist in transport of the tape across the tape head, and to provide proper tensioning of the tape.
Magnetic tapes are stored on reels, which are typically mounted within a cassette housing. The cassette provides mechanical protection for the tape, and facilitates ease of handling the tape. While some cassettes contain both the tape source reel and a take-up reel onto which the tape is wound as it passes over the tape head, another practice is to configure the cassette with only the tape source reel. In this latter configuration, a free end of the tape is connected to a take-up-reel which is part of the tape processing device. This single-reel cassette design reduces the storage area required to store the tapes, as compared to the storage area requirements for a two-reel cassette.
FIG. 1 shows a simplified plan view of a prior-art design of a tape processing apparatus 1 having a tape head 11, a first tape support guide 12, a second tape support guide 14, and a take-up-reel 16. A single-reel cassette 18 containing a source reel 15 of magnetic tape xe2x80x9cTxe2x80x9d can be mounted on the tape processing apparatus 1. The free end of the tape xe2x80x9cTxe2x80x9d is passed over the first tape guide 12, the tape head 11, and the second tape guide 14, and is then connected to the take-up-reel 16. Drive motors (not shown) control the winding of the tape xe2x80x9cTxe2x80x9d onto the take-up reel, or rewinding the tape onto the source reel 15. FIG. 2 shows a front elevation view of the tape processing apparatus 1 of FIG. 1. In FIG. 2, the tape xe2x80x9cTxe2x80x9d is shown in partial view to allow the face of the tape head 11 to be displayed. It is understood, however, that the tape xe2x80x9cTxe2x80x9d passes in front of the tape head in this view. FIG. 2 shows the tape guides 12 and 14. Each tape guide is typically provided with an upper flange 17 and a lower flange 18. The purpose of the tape guide flanges is to keep the tape guided into a relatively fixed position with respect to the tape head 11. As the tape moves past the tape head in either direction xe2x80x9cAxe2x80x9d or xe2x80x9cBxe2x80x9d, a tape head element 21 can magnetically encode (xe2x80x9cwritexe2x80x9d) data onto the tape, or it can read data from the tape, depending on how the element is electronically configured. It is possible to electronically configure a tape head element to perform both read and write functions merely by electronic circuit switching within the tape processing apparatus. The process and apparatus for recording magnetic data onto, and reading data from, a magnetic tape are well known in the art, and generally will not be discussed further herein.
In order to increase the density of data storage onto magnetic tape, the data can be recorded onto the tape in xe2x80x9ctracksxe2x80x9d or xe2x80x9cchannelsxe2x80x9d. In a linear tape drive, this essentially consists of segmenting the tape into a plurality of tracks, or horizontal zones, were data is recorded, and separating these data zones with zones where no data is recorded. This separation of the data zones (tracks) allows the data from one track to be distinctly read by the tape head element, without magnetic interference from an adjacent data track. In order to read or write data from or to a multi-track tape, the tape head element needs to be able to access the various tracks. Common practice is to provide the tape head with a plurality of tape head elements. Such a configuration is known as an element array, or merely an xe2x80x9carrayxe2x80x9d. One example of an 8-channel element array is shown in FIG. 2. The tape head 11 comprises a first array 22 having 8 elements, and an adjacent second array 23 also having 8 elements. The use of an element array allows data to be simultaneously recorded on, or read from, up to eight tracks. The use of two element arrays allows data to be recorded onto the tape by the first array, and then immediately read from the tape by the second array. In this manner the accuracy of data recorded onto the tape can be verified through comparison of the recorded and read data. By electronic switching within the tape drive, the functions of the arrays can be switched from recording data to reading data, and visa versa.
In one commercial example, a tape drive can have two tape head arrays of eight elements each. Each element is configured to record data onto a track which is 1500 microns (xcexc) high (in the vertical direction of the tape, i.e., in the direction perpendicular to the primary direction of travel of the tape). Each track is separated by a 28 xcexcband.
One drawback to tape drives is that the tape tends to xe2x80x9cwanderxe2x80x9d as it moves across the tape head in the primary direction. That is, viewing FIG. 2, as the tape 30 generally moves across the tape head in the horizontal direction, indicated by the arrow xe2x80x9cHxe2x80x9d, the tapes also tends to randomly move up and down in the vertical direction xe2x80x9cVxe2x80x9d. Such vertical movement is typically relatively slow as compared to movement in the horizontal direction, although notable exceptions do occur, as described further below. The effect of tape wander is that data recorded at a given location on a magnetic tape may not be readable if that location wanders away from the read-element when the tape is being read. Tape wander results from inconsistencies in the tape, such as thickness and tension, inconsistencies between tape reels, minor misalignment and tolerances of components within the tape drive itself, and a variety of other factors, collectively making it impractical if not impossible to eliminate tape wander through design changes. One prior art method for accommodating the effects of tape wander is to size the head elements and their spacing (in an array) within the limits of wander. That is, if a head element is big enough, and far enough away from a neighboring element, then the path of tape associated with the corresponding data track will always be in contact with the element, even as the tape wanders. However, this solution has the undesirable effect of reducing the density of data which can be stored on a tape of a given width, since the data channels or tracks are necessarily wider, and fewer tracks can thus be placed onto a tape of a given width.
Another known technique to eliminate the effects of tape wander is for the tape head to track the tape as the tape wanders. That is, by using a control system in conjunction with a tape head positioning device, the vertical position of the tape at any given time can be determined, and then the tape head can be moved up or down to keep the tape head in relative fixed position with the tape, even as the tape wanders. However, occasionally the tape will wander to the extreme edge of a tape guide. When this occurs, the edge of the tape hits the tape guide flange (see items 17 and 18 of FIG. 2), and the tape is radically moved in the opposite vertical direction back onto the tape guide. Most tape head tracking positioners are essentially incapable of following such radical movement within the necessary parameters. This has the detrimental effect of causing the tape elements to fail to accurately track a channel on the tape when the tape makes a radical change in position. The result is that if a tape jump of significant magnitude occurs when the tape head is processing data from or to the tape, some data may not be read from the tape, or some data recorded on the tape might be incapable of being read later. The obvious solution of removing the tape guide flanges is of course impractical, since the tape could then wander off the tape guide.
One solution is to use a high-speed (high bandwidth) tape head control positioner for the tape head. Such a positioner typically comprises a linear motor. However, one drawback to the using such a tape head positioning device is that with the use of a linear servo motor, the head is subject to the effects of shock and vibration in the chassis of the tape drive. This is due to the fact that the base of the linear motor, as well as the tape reel supports, are essentially fixed to the chassis of the tape drive. Any motion in the drive chassis will thus be imparted to these two components. However, the tape head in such a system is mounted to the slide portion of the linear motor, such that the tape head is essentially free-floating in the vertical direction and is only held in position by electromagnetic forces. Thus, the tape head will tend to stay in position as the tape chassis, and with it the tape, are moved as a result of shock and vibration imparted to the chassis. This has the detrimental effect of causing the tape elements to fail to accurately track a channel on the tape when a shock or significant vibration is imparted to the tape drive. The results can be unread or unreadable data, as described above.
An additional drawback to magnetic tapes is that as the tape is wound onto one of the tape reels, the tape can bump up against the upper or lower flange of the tape reel. This can cause damage to the tape, and so it is preferable to center the tape on the tape reel between the flanges of the reel. However, if the tape is perfectly aligned when it is wound onto one of the tape reels, yet another problem can occur. Due to a small increase in the thickness of the tape at the outer edges of the tape, when the outer edges are all aligned on the tape reel, the cumulative effect can be to produce a crinkling effect at the edge of the tape. This can result in mechanical damage of the tape, and is therefore to be avoided.
What is needed then is a tape drive which is capable of accommodating the effect of, or preferably minimizing the phenomenon of, tape wander. What is also needed is a way to more effectively wind magnetic tape onto a reel to prevent damage to the tape.
The present invention includes methods and apparatus for a magnetic tape processing device, or tape drive, which allow a magnetic tape to be positioned with respect to a tape head configured to write data to, or read data from, the magnetic tape. The invention comprises actively controlling the position of a tape guide used to support the tape as it passes over the tape head. By controlling the position of the tape guide, the tape can be caused to move in a direction perpendicular to the primary direction of transport of the tape past the tape head.
In a first embodiment, the invention comprises an apparatus for positioning a moving magnetic tape with respect to a tape head element in a tape head. The apparatus includes a first tape guide supported proximate to a first side of the tape head and mounted along a primary axis of the tape guide. For example, if the tape guide is cylindrical in shape, then the tape guide is mounted along the longitudinal axis of the tape guide. The first tape guide is configured to support the magnetic tape as the tape travels past the tape head. The apparatus also has a first tape guide tilt positioner configured to tilt the first tape guide. Preferably, the tape guide is configured to tilt towards or away from the magnetic tape, and thereby produce selective differential forces on the tape across its vertical height. In response to these forces, the tape will to move in a vertical direction (either up or down) until the vertical forces on the tape have been equalized.
When the response of the tape to movement of the tape guide can be modeled using an algorithm, the movement of the tape guide (direction and degree) can be controlled by an open-loop control system. Alternately, when the precise response characteristics of the tape to movement of the tape guide cannot be accurately predicted, a closed loop control system can be utilized to control the movement of the tape guide. Such a closed loop system can incorporate a tape position sensor or sensors and a feedback mechanism to provide tape position and movement information to the tilt positioner. This will be further described below.
The apparatus of the first embodiment can further include a first tape guide translation positioner configured to translate the first tape guide along the first tape guide longitudinal axis. This essentially provides for vertical movement of the tape guide along its longitudinal axis. Such movement of the tape guide imparts a drag force on the tape, causing the tape to move in the same vertical direction as the tape guide is moved. In one variation, the tilt positioner is replaced with the translational positioner, so that the tape guide is not configured to tilt, but to move translationally.
In one variation, the above apparatus can also have a second tape guide supported proximate to a second side of the tape head and mounted along the primary axis of second tape guide. The second tape guide is configured to support the magnetic tape as the tape travels past the tape head. This variation also includes a second tape guide tilt positioner configured to tilt the second tape guide in the same manner as the first tape guide can be tilted. The second tape guide can also be connected to a second tape guide translational positioner configured to translate the second tape guide along the second tape guide primary axis.
The apparatus of the present invention can thus include one or more tape guides, and each tape guide can be configured to tilt or move translationally (vertically), or to do both. In a configuration with two tape guides, the tape guides can be configured to tilt or translate independently of one another, or in unison with each other. The apparatus can include a tape guide positioning apparatus, such as a servo mechanism. The positioning apparatus can comprise a sensor for determining the vertical location of the tape at any given time, and an actuator for actuating the tape guide positioner(s). The servo mechanism can be configured to account for the rate and frequency of tape movement desired.
In a second embodiment, the invention includes a method for actively positioning a magnetic tape with respect to a tape head having head elements for at least performing one of writing or reading magnetically encoded data respectively to or from the magnetic tape. The method includes the steps of providing a first tape guide aligned along a primary axis and disposed proximate a first side of the tape head. The tape is then moved across the first tape guide and the tape head in a primary direction while the first tape guide is pivoted, to thereby cause the tape to move in a secondary direction essentially perpendicular to the primary direction. Preferably, the tape guide is tilted towards or away from the magnetic tape, and thereby a selective differential tension is created between the upper and lower edges of the tape. The tape will consequently move vertically (i.e., xe2x80x9cupxe2x80x9d or xe2x80x9cdownxe2x80x9d, relative to a primarily horizontal direction of travel) to relieve the differential forces in the tape, as described above.
Alternately, or in addition to the step of pivoting the first tape guide, the method can include the step of translationally moving the first tape guide along the primary axis while moving the tape across the first tape guide and the tape head in the first primary direction. This will also cause the tape to move in the secondary direction.
The method of the invention can further include providing a second tape guide disposed proximate a second side of the tape head opposite the first side of the tape head, and aligned along a primary axis of the tape guide. The second tape guide is then pivoted about the primary axis of the tape guide while moving the tape across the second tape guide and the tape head in the primary direction, to thereby further cause the tape to move in the secondary direction. As with the first tape guide, the second tape guide can alternately, or additionally, be moved translationally along the primary axis while moving the tape across the second tape guide and the tape head in the first primary direction. This will also cause the tape to move in the secondary direction.
The methods of the invention can further include the steps of providing a position sensor to detect the position of the tape relative to the tape head, and ceasing or initiate tilting, translating, or both movements of the first and/or second tape guides in response to the detected position of the tape.